Flutter
Flutter is an open-source UI software development kit created by Google for building natively compiled applications for mobile, web, and desktop from a single codebase. It uses the Dart programming language and provides a rich set of pre-designed widgets to create visually appealing, high-performance applications with a native look and feel on multiple platforms.
Developers should learn Flutter when they need to build cross-platform applications efficiently, as it allows for rapid development with hot reload, reducing time-to-market. It is particularly useful for startups and businesses aiming to deploy on both iOS and Android simultaneously without maintaining separate codebases, and for creating visually rich, custom UI designs that perform consistently across devices.